1st Lecture on Elements of groups 16, 17 & 18 | Chemistry Part I | 12th Std

1st Lecture on Elements of groups 16, 17 & 18
Chemistry Part I12th Standard
by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
1
st
Lecture on Elements of
groups 16, 17 & 18
Chemistry Part I, 12
th
Science
By
Rizwana Mohammad

2. Elements of groups 16, 17 and 18
Inthep-blockelementsthedifferentiatingelectron(thelastfilling
electron)entersthep-orbitaloftheoutermostshall.
Occurrence:
•Theelementsoxygen,sulphur,selenium,telluriumandpolonium
constitutegroup16,calledtheoxygenfamily.
•Largenumberofmetaloresareoxidesorsulphides.
•Group16elementsarealsocalledChalcogens.
•Oxygenisthemostabundantelementonearth.
•Oxygenforms20.95%byvolumeofairand46.6%bymassofearth's
crust.
•Sulphurforms0.034%bymassofearthcrust.
•Sulphuroccursmainlyincombinedformsassulphatessuchas
gypsum(CaSO
4,2H
2O),epsumsalt(MgSO
4,7H
2O),baryte(BaSO
4)
andsulphidessuchasgalena(PbS),zincblend(ZnS),copperpyrites
(CuFeS
2).

3. •Seleniumandtelluriumarealsofoundasmetalselenidesand
telluridesinsulphideores.
•Poloniumwhichisradioactiveisadecayproductofthoriumand
uranium.
•Fluorine,chlorine,bromine,iodineandastatineconstitutegroup17.
•Thesearecollectivelyknownashalogens(Greekhalomeanssalt,gene
meansborn),i.e.saltproducingelement.
•Halogensareveryreactiveduetohighelectronegativitiesandhence
theyarenotfoundinfreestate.
•Fluorineoccursmainlyasinsolublefluorides(FluorsparCaF
2,Cryolite
Na
3AlF
6,fluorapatite3Ca
3(PO
4)
2.CaF
2),andsmallquantitiesarepresent
insoil,freshwaterandplants,bonesandteethofanimals.
•Seawatercontainschlorides,bromidesandiodidesofNa,K,Mg,Ca.
ThedepositsofdriedupseabedscontainNaClandcarnallite,
KCl.MgCl
2.6H
2O.
•Seaweedcontains0.5%Iandchilesaltpetrecontains0.2%ofsodium
iodate.
•Astatineisradioactiveandhasahalflifeof8.1hours.

4. •Helium,neon,argon,krypton,xenon,radonconstitutegroup18.
•Allthenoblegasesexceptradonoccurintheatmosphere.
•Theirabundanceindryairis≈1%(byvolume),argonisthemajor
constituent.
•Heliumandneonarefoundinmineralsofradioactiveorigin,e.g.
pitchblende,monazite,cleveite.
•XeandRnaretherarestelement.
•Rnisadecayproductof
226
Ra.
Electronicconfigurationofelementsofgroup16,17and18:

5. Atomicandphysicalpropertiesofelementsofgroup16,17and18:
Atomicpropertiesofgroup16,17and18elements:
i.Atomicandionicradii:
Ingroup16,17,18atomicandionicradiiincreasedownthegroup,dueto
increaseinthenumberofquantumshells.
•Acrosstheperiodatomicorionicradiidecreasewithincreasingatomic
number.
•Group17elementshavethesmallestatomicradiiintheirrespectiveperiods.
ii.Ionizationenthalpy:
•Thegroup16,17,18elementshavehighionisationenthalpy.
•Theionisationenthalpydecreasesdownthegroupduetoincreaseinthe
atomicsize.
•Acrossaperiodionisationenthalpyincreaseswithincreaseofatomicnumber.
•Theelementsofgroup16havelowerIEvaluescomparedtothoseofgroup
15incorrespondingperiods.
•Itisduetoextrastablehalffilledelectronicconfigurationofp-orbitalsin
elementsofgroup15.
iii.Electronegativity:
•Inagroup(16,17,18)theelectronegativitydecreasesdownthegroup.
•Oxygenhasthehighestelectronegativitynexttofluorine.

6. •Halogenshaveveryhighelectronegativity.
•Fisthemostelectronegativeelementintheperiodictable.
Electrongainenthalpy:
•Inthegroup16and17electrongainenthalpybecomesless
negativedownthegroup.
•Ingroup16,oxygenhaslessnegativeelectiongainenthalpy
(-141kJ/mol)thansulphur(-200kJ/mol)duetoitssmallatomicsize.
•Ingroup17,Fhaslessnegativeelectrongainenthalpy(-333kJ/mol)
thanthatofchlorine(-349kJ/mol).
•Itisduetoitssmallatomicsize.
•Group18elementshavenotendencytoacceptelectronsbecause
oftheirstableelectronicconfiguration(ns
2
np
6
).
•Theyhavelargepositiveelectrongainenthalpy.

7. Physicalpropertiesofgroup16,17&18elements:
a.Group16elements(oxygenfamilyorchalcogens):
•Oxygenisagaswhileotherelementsaresolidsatroom
temperature.OandSaremetals,SeandTearemetalloids,Poisa
metal.
•Poisradioactivewithhalflifeof13.8days.
•Meltingandboilingpointsincreasewithincreasingatomicnumber.
•Alltheelementsofgroup16exhibitallotropy.
b.Group17elements(Halogenfamily):
•F,Claregases,BrisaliquidandIisasolidatroomtemperature.
•F
2isyellow,Cl
2greenishyellow,Br
2redandI
2isviolet,incolour.
•FandClreactwithwater.
•BrandIareonlysparinglysolubleinwaterandaresolublein
variousorganicsolventssuchaschloroform,carbondisulphide,
carbontetrachloride,hydrocarbonswhichgivecolouredsolutions.
•Bonddissociationenthalpiesofhalogenmoleculesfollowtheorder
•Cl–Cl>Br–Br>F–F>I–I.

8. c.Group18elements(Noblegases):
•Noblegasesaremonoatomic.
•Theyaresparinglysolubleinwater.
•Noblegaseshavelowmeltingandboilingpoints.Hehasthelowest
boilingpoint(4.2k)ofanyknowsubstance.